Fentanyl, or its pharmaceutically acceptable salt, is an opioid receptor agonist. A preferred pharmaceutically acceptable salt of fentanyl is fentanyl citrate. The chemical name of fentanyl citrate is N-(1-phenylethyl)-4-piperidyl)-propionanilide citrate (1:1), which has the following structural formula:

The approved products of fentanyl include tablets, lozenges (oral), transdermal patches and injectable solutions. Most of the injectable products of fentanyl currently available in the market contain sodium chloride, and these products, when intended for intravenous infusion, require dilution before administration using a suitable diluent, such as normal saline. For patients in need of therapy with fentanyl or a salt thereof, who also suffer from one or more conditions that are aggravated by increased plasma concentration of sodium ions, such as hypertension, hypernatremia, heart ailments such as congenital heart diseases, renal insufficiency, critical illness, and the like, it is important to avoid administration of sodium ions either as a constituent of the formulation or as a diluent. Because the currently available options involve the possibility of the use of preparations containing sodium ions, there always remains a chance that sodium ions may be accidentally administered to these patients. If proper care or attention is not taken and such patients are accidentally administered a preparation containing sodium ions, the patient's condition may be aggravated and become life threatening. There have been several reports of the accidental administration of sodium-containing preparations to patients who suffer from one or more conditions that get aggravated by an increased plasma concentration of sodium ions. Thus, there remains a need for a method of treating patients in need of therapy with fentanyl or a salt thereof, which method advantageously prevents or avoids the accidental infusion of preparations containing sodium to the patients who are in need of therapy with fentanyl or a salt thereof and who are also suffering from a condition which is aggravated by an increased plasma concentration of sodium ions.